Experimental study on flow boiling CHF of a helical finned rod under heaving conditions using simulant fluid R134a

Floating nuclear power plants (FNPPs) have garnered attention as a promising solution to provide clean energy and seawater desalination, addressing the energy needs of remote areas without access to the power grid. However, the oceanic environment can introduce variations in thermal-hydraulic phenomena, particularly affecting Critical Heat Flux (CHF). While few experimental studies have explored CHF under rolling and heaving motions, there is a need to expand the experimental database to encompass various test section geometries. This is necessary to elucidate the motion effect on the CHF mechanism and evaluate the applicability of helical finned fuel rods for FNPPs. Hence, the present study conducted experiments to assess the heaving motion effect on the flow boiling CHF of a helical finned rod. To simulate the heaving motion of ocean environments, the tests were performed on a heaving motion platform with a maximum acceleration of 0.6 g and periods ranging from 3 to 6 s. The CHF test loop utilized refrigerant R134a as the working fluid, with test conditions representative of pressurized water reactor operations. The test section featured a helical finned heater rod inside an annulus channel. CHF tests were conducted in conditions of both static and heaving motion to examine the CHF variation due to the oscillatory acceleration field. The results revealed that CHF could exhibit decreases of up to 5% or increases of up to 3%, depending on the specific thermal-hydraulic and motion conditions in the present test configuration. Notably, similar to previous experiments involving a bare heater rod, CHF reduction due to heaving motion was prominent in two specific areas: one where the critical quality neared zero and another where it exceeded 0.8. Additionally, an increase in CHF was observed around the critical quality of -0.2, a phenomenon not observed in bare rod tests. This suggests a unique mechanism associated with the helical fin structure. This study proposed CHF variation mechanisms for each region based on the experimental observations.